Australia: The Land Where Time Began

A biography of the Australian continent 

Cambrian Explosion - Characteristic Assemblage Types 

The chances of any fossils from more than 500 Ma being preserved is exceedingly low, made worse most by the organisms from the Early Cambrian being mostly soft-bodied. The conditions on the sea floor also vary over space and time so that the conditions conducive to the preservation of any fossils, but especially soft-bodied fossils, also vary over time and from place to place. The presence or absence of other organisms also affects the changes of an animal being fossilised. An example is the presence of algal mats that can help protect the remains of dead animals which gives them time to undergo the processes of fossilisation, remains that would otherwise be damaged or destroyed by scavengers and traces could be removed by the work of bioturbators. The chances of preservation of fossils is also affected by the chemical history of the sediments entombing the fossil remain. The sorts of biomineralised skeletons forming the bulk of the invertebrate record of the Phanerozoic, which represent a range of “normal” modes of preservation, are contained at many Cambrian localities. Skeletal fossils from the Neoproterozoic or Cambrian, as with most cases throughout the Phanerozoic, have often had their minerals replaced by different materials.

Distinctive preservation styles characterise unusual body fossil assemblages that the authors¹ describe as particularly striking enhance the record of the Cambrian Explosion. Original organic carbon films represent many of the fossils from this time (Butterfield, 1990, 1995). This type of preservation is named for the Burgess Shale Fauna of British Columbia, Canada and is also found in the Chengjiang Fauna from Yunnan, China, that is earlier (Butterfield, 1990, 1995; Gaines, Briggs and Zhao, 2008). In this style of preservation the usual processes of bacterial decay are suppressed, the authors¹ suggesting probably by anoxic conditions. A large proportion of fossils reviewed in the chapter are from these and other, similar faunas. Another especially important type of preservation found in these deposits from the Early Cambrian is of very small shells found in phosphate deposits, the shells being either originally phosphatic or their constituents have been replaced by phosphate minerals during fossilisation. A different fraction of a biota is preserved by different accumulation and fossilisation histories of organisms. A range of these histories have been preserved in the known deposits from the Early Cambrian. The richness of these faunas gives some indication of how much is missing from times where only “normal” preservation modes are represented.

Sources & Further reading

1. Erwin, Douglas H., & Valentine, James W., 2013, The Cambrian Explosion: The Construction of Animal Biodiversity, Roberts & Co., Greenwood Village, Colorado